It has long been known to improve the corrosion resistance of flat steel products 100, such as steel strips or steel sheets, by coating them with a zinc alloy. In practice, this mostly takes place when the steel product comes out of a furnace and is introduced into a zinc alloy bath 11, as is indicated with the aid of an exemplary apparatus 200 in FIG. 10. In order to protect the flat steel product 100 from oxidation, the product typically passes through a sleeve 12 into the zinc alloy bath. In the bath 11, the flat steel product 100 is redirected by a roller 13, and moved upwardly out of the bath 11. Upon leaving the bath 11, the adhering melted film of the steel product 100 is stripped off by means of a gas stream from the nozzles 14 of a jet stripping unit to the target size, and the flat steel product 100 is then transferred to a cooling zone 15. Upon leaving the zinc alloy bath 11, the band-shaped flat steel product 100 rips with it a quantity of alloy that is dependent upon conveyor speed. The quantity of alloy is a multiple of the desired alloy plating. With the directed stream (nitrogen, air, or a mixture thereof at 10-70° C.) from the nozzles 14 (preferably wide flat nozzles are used), so much of the melted alloy is stripped off until the desired plating remains on the flat steel product 100. The coating thickness is among others a function of the conveyor speed, of the blow-off pressure and of the spacing of the nozzles 14 from the band-shaped flat steel product 100. The coating thickness of the zinc alloy on the flat steel product 100 can therefore be influenced by the nozzles 14. In addition, however, inherent properties of the alloy composition play a role. This continuously operating process is generally called hot-dip coating.
For a long time, so-called Zn—Mg—Al-coating systems are used, which develop an outstanding corrosion protection effect even at low coating thicknesses. The metallurgical performance of such a complex Zn—Mg—Al-coating system in a hot-dip bath 11, from a thermodynamic point of view, is only achieved with effort and simplifying assumptions have to be made in order to be able to simulate the hot-dip bath 11 itself and the covering of the flat steel product 100 with a protective coating in such a hot-dip bath 11. This is partly because the process of hot-dip coating is a dynamic process in which the flat steel product 100 is introduced into the bath 11 and removed from the bath 11 in a continuous process. Moreover, the composition of the bath 11, i.e., the locally present concentrations of the individual alloy components, can momentarily change locally and also the temperature distribution can vary slightly. There are already numerous investigations and patent applications, each dealing with partial aspects of the Zn—Mg—Al-coating systems in the hot-dip bath 11. Examples are mentioned in the following:                GB 1,125,965: This older patent document describes a Zn—Mg—Al-coating system in which the following large range for alloy compositions (in weight percent) are defined: 1<Mg<4 and 0.05<Al<5. Specific embodiments and the technical teachings of this older patent document refer either to Zn—Mg—Al-coating systems and hot-dip baths whose alloy has specifically one of the following alloy compositions (1. to 5.):                    1. Mg=2 weight %, Al=4 weight %;            2. Mg=2.4 weight %, Al=3.2 weight %;            3. Mg=2.4 weight %, Al=3.8 weight %;            4. Mg=2.49 weight %, Al=4.39 weight %;            5. Mg=2.5 weight %, Al=4.5 weight %;                        or to Zn—Mg—Al-coating systems and hot-dip baths whose alloy has specifically one of the following alloy compositions (6. to 9.):                    6. Mg=2.77 weight %, Al=0 weight %;            7. Mg=2.97 weight %, Al=0.12 weight %;            8. Mg=3 weight %, Al=0 weight %;            9. Mg=3 weight %, Al=0.2 weight %;                        WO2006/002843: This patent application describes a Zn—Mg—Al-coating system in which the alloy composition (in percent weight) is defined as follows: 0.3<Mg<2.3 and 0.6<Al<2.3. According to the teachings of this patent application a relatively large window extending between 0.3 weight percent and 2.3 weight percent is spanned for the magnesium component. In order not to adversely affect weldability, the teachings of this patent application state that the aluminum component should be set at a maximum of 2.3 weight percent.        EP 1 621 645 A1: This patent application describes a Zn—Mg—Al-coating system in which aluminum and magnesium in a ratio of 1:1 are used. This document explains that the sum of these alloy elements should not be too high because of the slag formation in the bath. The technical teaching of EP 1 621 645 A1 says among other things that beyond 2.3 weight percent aluminum and 2.3 weight percent magnesium one obtains an increasingly brittle coating in which the surface quality is noticeably deteriorated. Therefore, according to this patent application, a range between 0.6 and 1.3 weight percent of aluminum and 0.6 to 1.3 weight percent of magnesium is proposed.        WO 2012 091385 A2: This patent application also describes a Zn—Mg—Al-coating system wherein here besides Zn—Mg—Al a fourth element (for example silicon or lithium) is added to the bath. According to this patent application [Al/(Al+Mg)] should lie in the range between 0.38 and 0.48. This range information can be converted into the following statement: 0.61*Mg<Al<0.77*Mg. From this follows that the technical teaching of WO 2012 091385 A2 states that always more magnesium than aluminum should be present in the melt.        EP 1857566 A1: This patent application describes again a Zn—Mg—Al-coating system, in which further substances (for example, Pb, Si and others) are added in small quantities. The technical teaching of this document states that above all an alloy bath is preferred that contains between 0.15 and 0.4 weight percent aluminum and 0.2 to 2.0 weight percent magnesium. This patent application describes that one achieves an optimal combination of high corrosion resistance and optimized weldability by application of the described technical teachings.        EP 2119804 A1: This patent application also describes a Zn—Mg—Al-coating system, in which additional substances in small quantities (up to 0.3 percent weight) are added. The technical teachings of this document state that above all an alloy bath is preferred that contains between 2 and 8 weight percent aluminum and 0 to 5 weight percent magnesium. The task of this patent application is the reduction of the waviness of the solidified metallic protective coating.        The technical publication “Solidification Structure of the Coating Layer on Hot-Dip Zn-11% Al-3% Mg-0.2% Si-coated Steel Sheet” by K. Honda et al., Materials Transaction, Vol. 49, Nr. 6, 2008, pages 1395-1400, describes a Zn—Mg—Al-coating system that is produced from a bath with 11 weight percent aluminum, 3 weight percent magnesium and 0.2 weight percent silicon. It is reported that with the aid of the solidification structure it could be determined that—unlike in the state of equilibrium—MgZn2 instead of Mg2Zn11 could be observed. Apparently, in the particular experimental set-up and under the specified conditions MgZn2 is formed as a metastable structure (called Laves-Phase), whereas Mg2Zn11 does not. Additional information about this topic can be taken from the thesis of E. De Bruyker on “Zn—Mg—Al Alloy Coatings: thermodynamic analysis and microstructure related properties”, Dissertation, Univ. Gent, 2006.        There are also technical approaches, which are mainly applied in products from Asia, in which the eutectic solidification point is not reached over a eutectic valley. These approaches, however, lie typically in the hypereutectic range with over 5 weight percent per alloy element. From European Patent Application EP 1 466 994 A1 for example such an approach is known that produces a metal product whose Zn—Mg—Al protective coating contains between 2 and 19 weight percent aluminum and between 1 and 10 weight percent magnesium.        
Besides the pure protection against corrosion, there are however always progressing requirements in terms of surface quality of zinc-coated flat steel products. Above all, the automotive industry, but also the construction sector, expect products that satisfy the highest requirements towards surfaces.
The application of a zinc-based protective coating is a very dynamic process, which especially on a large industrial scale is determined by numerous parameters and influential factors. In the last few years various attempts were undertaken to operate the hot-dip zinc coating apparatus (as for example the apparatus 200 shown in FIG. 10) such that the quality of the surface provided with the zinc-based protective coating could be increased with consistently good protection against corrosion by the metallic coating. In addition, the ternary Al—Mg—Zn phase diagram of a ZnAlMg-melt presents a very complex system with numerous intermetallic phases including binary as well as ternary type. Providing homogeneous surfaces is therefore not trivial.
The main problems with this are often surface flaws that may occur during solidification of the ZnAlMg protective coating due to selective oxidation of the melt film.
Further aspects, which should be considered in providing of a suitable ZnAlMg protective coating, are the economics, the sensible use of valuable resources, and above all the energy expense, which must be made in production.
It is therefore the object to provide a method as well as the corresponding flat steel products which have corrosion technically an especially durable and robust protective coating, whereby the surface of the protective coating should be especially homogeneous and smooth. A surface quality is strived for that meets the highest requirements of the customer.
Additionally, this method should be as little as possible energy-consuming, cost-effective, simple and reproducible.